On opening this thread, an ad for an epoxy gel appeared.
Being in the marine industry, repairing and refinishing boats, I would suggest not using any polyester resin formulations since the shrinkage that occurs upon curing is significant, and could result in a plug that falls out.
Epoxy formulations have much less shrinkage, almost zero in some cases and provide much better adhesion . Resilience is much better than polyester as well, and probably a significant factor if you are planning to use a sledge hammer on your balls .
Epoxy is more expensive, but if you buy a small portion of the laminating epoxy like the West system and mix it with an inexpensive inert filler that you might buy at your local marine store, you can increase the volume of filler 3 or 4 times without significantly affecting the desired properties of the epoxy resin.